A DMS filter with three transducers is disclosed in EP 1394940, where an electric port of the filter is symmetrized by a transducer divided into two partial transducers connected in series. The partial transducers have a common bus bar which is connected to ground via opposing bus bars of the transducer connected to the asymmetric electric port. A floating metal structure is provided in one of the transducers to compensate for the undesired asymmetry.
From FIG. 6 of the publication EP 0605884 A1, another symmetrical/asymmetrical DMS filter, with a total of seven transducers, is known, in which input and output transducers are arranged alternately: three parallel-connected input transducers are connected to an asymmetric electric input port, and each of two parallel-connected output transducers are connected to a terminal of a symmetric output port.
A symmetrical/asymmetrical DMS filter is known from FIG. 7 of EP 0605884, in which, in contrast to FIG. 6 of EP 0605884, the symmetrization of the input port is achieved by subdivision (vertical splitting, V-split) of the middle input port into two series-connected component transducers.
Additional DMS filters are known from the publications U.S. Pat. No. 3,582,840, U.S. Pat. No. 5,694,096, U.S. Pat. No. 5,770,985 and U.S. Pat. No. 4,492,940.
Reflector losses of a DMS filter can be reduced with an increasing number of transducers connected in parallel that are associated with an input or output port. On the other hand, however, the parasitic capacitances of the filter arrangement increase with the number of feed lines elements that are necessary to transducers, and matching at the input and output ports deteriorates accordingly. With the parallel connection of additional input and output transducers, the aperture of the acoustic track becomes smaller and could be unfavorable to maintain a given impedance level at the input or output port. In the transition from an arrangement with three transducers to an arrangement with six transducers, for instance, the aperture is reduced by a factor of two. Thereby, one obtains a reduction by a factor of two of the finger resistances, which increase proportionally to the aperture. The finger resistances are even reduced by a factor of four, since the transducers are connected in parallel. This is of advantage for HF (high-frequency) filters, where small finger heights and widths lead to large finger resistance losses. The aperture should not be reduced too much, however, because it is known that then transversal 2D (2D=two-dimensional) losses increase. A critical value for the minimum aperture is roughly 20λ. The length of a filter increases from the use of larger numbers of transducers in the DMS track, one external ground connection such as one bond wire or one bump per transducer being necessary in case of additional transducers, which increases the surface area of the overall arrangement.